The present invention relates to a plasma processing apparatus and a plasma processing method for plasma-processing a semiconductor substrate such as a silicon wafer.
In manufacturing processes of silicon wafers for semiconductor devices, thinning processing for decreasing the thickness of a substrate has come to be performed increasingly as the thickness of semiconductor devices has decreased. In the thinning processing, after formation of circuit patterns on the front surface of a silicon substrate, the back surface, that is, the surface opposite to the circuit formation surface, is ground mechanically. After the mechanical grinding, plasma processing is performed to etch away a damaged layer that has been formed on the ground surface of the silicon substrate by the grinding.
In the plasma processing, the silicon wafer is held in a state that its circuit formation surface faces the mounting surface of a substrate mounting portion because the processing subject surface (i.e., back surface) of the silicon wafer needs to be located above. To prevent the circuits from directly touching the mounting surface and thereby being damaged, a protective film is stuck to the circuit formation surface.
One method for holding such a silicon wafer is a method utilizing electrostatic absorption. In this method, a silicon wafer is placed on a substrate mounting portion in which the surface of a conductor is covered with a thin insulating layer. A DC voltage is applied to the conductor to make the surface of the substrate mounting portion an electrostatic absorption surface. The silicon wafer is held by the substrate mounting portion by Coulomb force that acts between the silicon wafer and the conductor that is located under the insulating layer.
However, where a silicon wafer to which a protective film (mentioned above) is stuck is held by electrostatic absorption, the Coulomb force acts on the conductor via not only the insulating layer but also the insulative protective film and hence the electrostatic absorption force is weaker than in a case that the silicon wafer is in direct contact with the electrostatic absorption surface (i.e., without intervention of a protective film); sufficient holding force may not be obtained.
Further, since the entire surface of the substrate mounting portion is covered with the insulating layer, in a state that the silicon wafer is mounted thereon, the silicon wafer is not in direct contact with the conductor of the substrate mounting portion and hence the conductivity of heat from the silicon wafer to the substrate mounting portion is low. This makes it difficult to cool the silicon wafer by removing heat from it efficiently during plasma processing. If the substrate temperature increases excessively, the protective film that is stuck to the substrate receives heat damage. In view of this, in setting plasma processing conditions, the output power for plasma processing of a power source needs to be set low, that is, the output power is restricted by the substrate cooling ability of the substrate mounting portion. As described above, where the processing object is a semiconductor substrate having an insulating layer (protective film), it is difficult to increase the cooling efficiency while maintaining sufficient electrostatic holding force; it is difficult to realize plasma processing having a high etching rate.